Certain processes, such as combustion of carbon containing fuels, produce gaseous emissions of carbon dioxide (CO2). CO2 has been identified as a “greenhouse” gas, which appears to contribute to global warming. Because of its status as a “greenhouse” gas, technologies have been developed to prevent large quantities of CO2 from being released into the atmosphere from the use of fossil fuels.
Chemical looping combustion (CLC) is a combustion technology that provides efficient CO2 capture and processing. CLC provides for inherent separation of CO2 produced during oxidation of carbon containing fuels thereby creating a more concentrated stream of CO2. By increasing the concentration of the CO2 in the flue stream as part of the combustion technology, the energy and capital expenditures required to separate CO2 after combustion for capture and storage are substantially reduced.
CLC technology generally involves use of an oxygen carrier which transfers oxygen from air to a fuel, thereby avoiding direct contact between air and the fuel. Two inter-connected reactors, typically fluidized beds, are used in the process: a fuel reactor and an air reactor. The fuel is introduced in the fuel reactor, which contains the oxygen carrier such as a metal oxide, MeO. Depending on the fuel and the metal oxide, the fuel and the metal oxide may react according to the following reaction:(2n+m)MeO+CnH2m→(2n+m)Me+mH2O+nCO2 (oxidation of fuel)An exit gas stream from the fuel reactor primarily contains products from oxidation of the fuel, H2O and CO2. A stream consisting of a high concentration of CO2 may then be obtained by condensing the H2O contained in the exit gas stream of the fuel reactor.
A reduced metal oxide, Me, formed as part of fuel oxidation reaction, may be transferred to the air reactor where it may oxidize according to the following reaction:Me+½O2→MeO (oxidation of metal oxide)A flue stream exiting the air reactor consists primarily of non-reactive components of air, such as nitrogen, the metal oxide and some unused oxygen. Through the use of the oxygen carrier to deliver oxygen to the fuel reactor, the non-reactive components of air are expelled from the system as they exit the air reactor and are never introduced into the fuel reactor. Therefore, the products of combustion, primarily CO2 and H2O, are not diluted by non-reactive components of air in the flue stream of the fuel reactor.
Depending on the conditions and materials used, combustion of the fuel in the fuel reactor may be incomplete. Incomplete combustion may cause unburnts, such as hydrogen, methane, and carbon monoxide, to be present in the flue stream of the fuel reactor. In order to reduce or eliminate the unburnts from the flue stream, the unburnts are typically oxidized in a post combustion unit after combustion in the fuel reactor. The unburnts should not be returned completely to the fuel reactor for combustion because this may lead to an accumulation of non-reactive matter within the combustion system.
One of the difficulties with CLC is that the post combustion unit requires pure or enriched oxygen gas for oxidation of the unburnts. If air was added to the flue stream exiting the fuel reactor for purposes of post combustion oxidation, the benefits of CLC would be lost because the non-reactive constituents of air would be added to the flue stream. Accordingly, post combustion oxidation requires the addition of pure or oxygen enriched gas, which is expensive both in terms of energy consumption and capital costs. Moreover, depending on the amount of unburnts requiring oxidation, combustion in pure or enriched oxygen may lead to strongly elevated temperatures, requiring cooling. Accordingly, there is a need for an improved method and apparatus for more efficient treatment of unburnts.